The present invention is directed to electrochemical cells and more particularly to a method of preparing a carbon black active layer for a cathode which can be used in a chlor-alkali cell. Chlor-alkali cells are well-known cells used in industry. In a chlor-alkali cell, an electric current is passed through a saturated brine (sodium chloride salt) aqueous solution to produce chlorine gas and caustic soda (sodium hydroxide). The caustic environment present in chlor-alkali cells presents a very corrosive atmosphere for the anode and cathode. Various attempts have been made to achieve cost savings in respect of the electrical energy necessary to operate chlor-alkali cells. An unwanted by-product of the production of chlorine and caustic soda is the production of hydrogen gas at the cathode of the chlor-alkali cell. It has been estimated that approximately as much as 25 percent of the electric power required to operate chlor-alkali cells is used in the unwanted production of hydrogen at the cathode. Hence if the production of hydrogen gas can be eliminated, substantial economic savings in the cost of the electric power required to operate chlor-alkali cells can be achieved. Oxygen (air) cathodes eliminate the production of hydrogen at the cathode and force the formation of hydroxide ions.
One known form of cathode active layer, viz., layer which contains the active component or material, is an active cathode layer containing carbon black. Due to the very corrosive nature of the catholyte liquor, viz., the liquid medium with which the cathode is in contact in such chlor-alkali cells, various attempts have been made to reduce the wettability of the carbon black which constitutes the active component. The caustic soda has a tendency to wet the pores of the carbon black thereby reducing its efficiency and activity. Consequently, various materials have been employed to reduce the wettability and thereby increase the length of time for which the carbon black is effective.
Polytetrafluoroethylene (PTFE), in particulate form, has been employed in the carbon black active cathode layer in order to impart hydrophobicity to the carbon black cathode. Some cathode structures utilize layers of polytetrafluoroethylene, per se, with or without pore-forming materials to form protective or backing sheets, viz., protective sheets to further increase the hydrophobicity of the carbon black cathodes on the air side. Pore-forming agents are customarily utilized to impart porosity to the overall cathode structure so as to enhance contact between oxygen (or air) and catholyte, which may fill the larger pores. Of course, when water-soluble pore-forming agents are employed to produce such backing layers (which are subsequently secured to the active carbon black-containing layer to form a laminated assembly cathode, e.g., by lamination using heat and pressure), the pore-forming agent must be removed after formation of the laminated cathode. It was discovered that after laminating such carbon black (active) layer to the leachable pore former - PTFE layer using heat and high pressure, e.g., temperatures ranging from about 110.degree. to 125.degree. C. and pressures ranging from about 4 to about 10 tons per square inch, severe blistering of the carbon black-containing active layer occurred during the water washing step. This blistering in essence destroyed the structural integrity of the laminated cathode.